Electrical Noise - Sleeve vs Ball Bearing 1

[pugap]

We have an application utilizing a PSC motor with a type of triac type speed controller. The application is a motor in a blower, and the speeds run dictate that a ball bearing design should be used. Situation is that if a motor with a ball bearing construction is used, the motor is noisy (low electrical hum - typical of saturation with a speed controller). However, if we change to a rigid sleeve construction (motor is identical electrically), there is no noise. Vibration pickups have confirmed both contructions transmit the same vibration spectrums, but a sound spectrum shows the sleeve bearing does transmit sound differently.
One theory is that there is a current being induced in the shaft. Would a sleeve bearing shield this current, thus creating a "quieter" motor? Or s it more of a dampening effect on transmitting the motor vibration to the blower assembly?

 

[Skogsgurra]

I have run lots of shaded pole and split phase AV motors on triac controllers and I do not think that I would characterize the sound as "typical of saturation with a speed controller" if the controller works symmetrically (identical firing angles in quadrants I and III).

The difference in firing gate current levels that exists in QI and QIII does sometimes produce a hum with same frequency as mains frequency. So, if your sound has lots of 50 (or 60) Hz, you can be sure that it is a malfunction in the controller. If you, on the other hand, have a dominating frequency twice the mains (100 or 120 Hz), then you have a purely magnetic/mechanic hum.

Are motors really identical? Even if nameplate data are equal, the electrical characteristics may differ. Especially the leakage inductance, which plays a role when determining di/dt. The di/dt says how fast the holding current is reached and that may influence operation of the triac/motor combination.

A very simple test is to put an incandescent lamp parallel to the motor. It makes the triac well behaved and should eliminate the hum if it is from assymetric triac characteristics. But not if the firing pulses are asymmetrical. Check that with a scope.

The idea that something is induced in the shaft is valid for much larger machines, hundreds of HP. That will not be a problem in machines from FHP to tens of HP. And the current would not be possible to hear. There are many other forces in play that are much more dominant.

Gunnar Englund

http://www.gke.org

 

[pugap]

The motors are identical in every way (same stack, same winding design, etc) except for the bearing system. The application required that the original design (sleeve) be changed to a ball bearing.
Magnetic saturation of the steel is fairly common with PSC motors (though typically easy to avoid), but is more common in variable speed drive applications. Hook up a motor to a dimmer switch type control, turn it down, and you will hit a point where the electrical hum picks up. A simple fix is usually increasing the stack of the motor to decrease the magnetic flux density. The odd thing in this application is the bearing system seems to be dampening 'something'. Agree that induced currents are more typical in larger motors, which is what's troubling with this application.

 

[Skogsgurra]

When you "dim" the motor you decrease the voltage and that should not lead to a point where the iron saturates - unless the triac triggers unsymetrically in QI and QIII. The saturation is then a natural consequence of the DC component in the motor current. So the problem is still in the controller, not in the motor.

Have you actually changed bearings in one and the same motor? Or are you comparing two individuals with different bearings?

I may not have understood your root problem. Is it the hum or the fact that the hum sounds differently in the two motors?

Gunnar Englund

http://www.gke.org

 

[pugap]

The controller chops the waveform so that it is no longer sinusoidal - very common situation. The more choppy, the more saturation is induced. I think you are thinking of a variac, which controls the line voltage.
The situation is application specific, which means all motors, not two individual motors. The question is why is the sleeve bearing constrution sound different than the ball bearing.

 

[Skogsgurra]

No pugap. I have been designing triac and thyristor controllers for decades. I know exactly what a phase-controlled waveform looks like. I also know what happens when the triggering is insufficient and QI and QIII are different. What happens is that you get a DC component and that is what is saturating your iron. Low voltage does not saturate iron - high voltage does.

You have to believe me. I know about these things.

Gunnar Englund

http://www.gke.org

 

[Skogsgurra]

Found this for you:

http://www.st.com/stonline/products/literature/an/3565.pdf

This link tells all you need to know about triacs, inductive loads, quadrants and saturation.



Gunnar Englund

http://www.gke.org

 

[pugap]

The wave forms I typically come across, and is true in this application, have a lot of ringing and are not clean. It's just the way it is (sounds like a business opportunity for you ). Thus we deal with noisy motors due to saturation from the dirty waveform. We have no say or control over the design of the controller use, but we have to deal with it's affect on the motor. If the controller is removed and the motor is run at the same RMS voltage - no noise regardless of the bearing system.
Back to the root question - why is the sleeve bearing dampening out the noise?

 

[Skogsgurra]

That is most likely a result of the tighter tolerances in the ball bearing. The sleeve bearing has a rather thick oil film, which acts as a damper.

I think that you should try the tip with the parallel lamp. It very often makes the controller behave better. It is not a solution, but if you get a better result, you can go back to the controller designer and tell him to control the triggering better.

BTW, is the controller processor-based or using a diac or UJT? Processor-based controllers often perform a lot better than discrete component controllers.

Gunnar Englund

http://www.gke.org

 

[ozmosis]

you are a credit to the profession Gunnar and epitomise what forums like this are all about!